The present invention relates to a method for adjusting an equalizer having a plurality of variable-type band-division equalizer sections, and in particular, relates to an improved equalization method which provides the optimum setting for all equalizing sections taking into account the interaction among all equalizing sections (hereinafter called "interaction"), and an improved equalization method in which any difficulties arising from such interaction are eliminated by giving a "set" command through the calculation of the interaction.
An equalizer having a plurality of variable-type band-division equalizer sections for equalizing the delay characteristics of a transmission circuit is well known. When the required characteristics of the transmission circuit are severe, the total equalization characteristic of an equalizer is equal to the sum of the characteristics of each individual equalizer section. For instance, when the required characteristic is flat, the equalizer must have the inverse characteristics of the transmission circuit. The equalizer mentioned above is called a variable-type band-division equalizer.
FIG. 1 shows an example of an equalizer of this type which is employed for delay equalization for voice bands. In FIG. 1 showing a circuit unit 1 and a setting unit 2, the input signal is, after being amplified to a desired level by an amplifier AMP, equalized through a low band circuit LB, a high band circuit HB, and equalizing sections, SEC 1 (0.6 KHz) through SEC 12 (2.8 KHz), which are provided at intervals of 200 Hz. The setting of the variable characteristics is accomplished by switches for LB and HB and by variable resistors, RF 1 through 12 which connect to the equalizing sections, SEC 1 through 12.
FIG. 2 shows an example of some of the variable characteristics of SEC 8 (f=2.0 KHz) which shows clearly the change of delay time depending upon the resistance of RF8, such as r 1, r 2, r 3 . . . . However, while it has the advantage of being adjustable for any desired characteristic, an equalizer of this type has the disadvantage of requiring skill and a considerable amount of time for adjustment. As is clearly seen from the variable characteristics shown in FIG. 2, the variable characteristics of one of the equalizing sections interact not only upon the section adjacent thereto but also upon all the other sections, and more over such interaction is of such a nature as to change the characteristics asymmetrically and non-linerally. This causes the above difficulty in the adjustment of an equalizer.
FIG. 3 shows an example of the interaction between one of the equalizing sections, SEC 8 (f=2.0 KHz), and the other sections, SEC 6 (f=1.6 KHz), SEC 7 (f=1.8 KHz), SEC 9 (f=2.2 KHz) and SEC 10 (f=2.4 KHz). It should be noted that the interaction I.sub.87 and I.sub.89 respectively upon SEC 7 and SEC 9 by SEC 8 and I.sub.86 and I.sub.810 respectively upon SEC 6 and SEC 10 by SEC 8 are all asymmetrical to the characteristics of SEC 8 itself (I.sub.88) and are non-linear.
For adjustment of the equalizer of the above type, a method by means of a delay measuring apparatus as schematically illustrated in FIG. 4 (A) and FIG. 4 (B), has conventionally been employed. For transmission equalization in FIG. 4 (A), a transmitting station 10 sends a sweep signal, (f=0.3 KH.sub.z through 3.4 KHz), from the sweep oscillator 11a in a delay distortion measuring apparatus 11, to a line 15, and in a receiving station 20 which receives the signal, the value of delay distortion in the line 15, is detected by a detector 21a, and is sent back through the re-modulator 21b in a delay distortion measuring apparatus at a given frequency, for example, f=2.0 KHz, to the transmitting station 10, in which this information is detected by a detector 11b and is indicated on the screen of an indicator 12. Thus, the adjustment of the equalizer 13 and/or 23 is manually accomplished.
Further referring to FIG. 4 (A), numerals 13 and 23 are equalizers, and 15 is an international data line.
Reception equalization as illustrated in FIG. 4 (B) is accomplished by a procedure similar to that for the transmission equalization in FIG. 4 (A). In FIG. 4 (B), 10' is a transmitting station, 20' is a receiving station, 15' is an international data line, 11a' is the sweep ocillator section of a delay distortion measuring apparatus, 12a' is the detector section of the delay distortion measurer, 22 is a display device, and 13' and 23' are equalizers. It is the advantage of the above equalization method that high precision of equalization can be attained. However, the sweep step requires not only a long time of operation but also a certain skill. Furthermore, remote control of the measuring devices, that is to say, the control of the apparatus at the receiving station from the transmission station, and the control of the apparatus at the transmission station from the receiving station, is necessary when an international data line is to be equalized. However, due to the limit of operation hours in a day, and/or the inadequacy of apparatuses at both stations, the adjustment of an equalizer by the method in FIG. 4 (A) or FIG. 4 (B) is very difficult.